Enantiomerically pure β-amino acids are valuable building blocks for novel therapeutics agents that possess a wide range of biological activity. Although a number of biocatalytic routes have been developed for their preparation, no single method has emerged as being universally applicable. Similarly, few chemo-catalytic routes to β-amino acids have been developed, most requiring stoichiometric quantities of chiral auxiliaries.
Dihydropyrimidinases and hydantoinase are possible candidates for the biocatalytic synthesis of amino acids or their precursors. Gaebler and Keltch first reported hydantoinase cleaving activities in 1920s (Gaebler, O. H.; Keltch, A. K. On the metabolism of hydantoins and hydantoic acid, 1926; Vol. 70). It was initially suggested by Eadie et al. in the 1950s that microbial hydantoinases were identical to animal dihydropyrimidinase (Eadie, G.; Bernheim, F.; Bernheim, M. Journal of Biological Chemistry 181: 449-458, 1949). Dihydropyrimidinase enzymes, isolated from calf liver and plants, catalysed the hydrolysis of dihydrouracil and dihydrothymine into the N-carbamoyl-β-alanine and N-carbamoyl-2-methyl-β-alanine, respectively. These enzymes also cleaved (R)-5-monosubsitituted hydantoin into (R)—N-carbamoyl-amino acid. Recent literature generally proposes that D-hydantoinase from microbial sources can be considered to be the counterpart of animal dihydropyrimidinase, with Nonaka and co-workers, suggesting an evolutionary relationship between these two enzymes (Hamajima, N.; Matsuda, K.; Sakata, S.; Tamaki, N.; Sasaki, M.; Nonaka, M. Gene 180:157-163, 1996). Syldatk et al. conclude that dihydropyrimidinases and hydantoinases are not necessarily the same enzyme (Syldatk, C.; May, O.; Altenbuchner, J.; Mattes, R.; Siemann, M. Applied Microbiology and Biotechnology 51:293-309, 1999). The different entantioselecivities of hydantoinases are often used to group them, according to their specificity, as D-, L-, or nonspecific (Ogawa, J.; Shimizu, S. Journal of Molecular Catalysis B: Enzymatic 2:163-176, 1997).
Problems arising from the naming system used for the hydantoinase and dihydropyrimidinase enzymes are further aggravated by the fact that often, especially in earlier journals, the terms were used interchangeably. Amidohydrolases, also referred to as cyclic amidases [E.C.3.5.2], are a group of more than 14 enzymes all acting on cyclic amide rings and containing a number of highly conserved regions and invariant amino acid regions (Kim, G. J.; Cheon, Y. H.; Kim, H. S. Biotechnology and Bioengineering 1998, 61, 1-13). Comprised in this group are carboxylmethylhydantoinase [E.C.3.5.2.4], allantoinase [E.C.3.5.2.5], 1-methylhydantoinase [E.C.3.5.2.14] and carboxyethyl-hydantoinase, all of which are technically the only hydantoinases, as their substrates are naturally occurring hydantoin derivatives.
Other enzymes which fall into the wider grouping of cyclic amidases include dihydroorotase [E.C.3.5.2.3] and dihydropyrimidinase [E.C.3.5.2.2], the latter of which is commonly referred to as D-hydantoinase, due to its ability to hydrolyse (R)-5-monosubstituted hydantoin derivatives. This superfamily of proteins most likely evolved in prehistoric earth, when N-carbamoyl-amino acids are hypothesised to have been the original synthons of prebiotic peptides.
The use of hydantoinases for the enantioselective hydrolysis of racemic mixtures of 5-substituted hydantoins (R)-1 and (S)-1 to their corresponding N-carbamoyl derivatives (R)-2 and (S)-2 is well established (cf. Scheme 1 below) and described in literature (Morin, Enzyme Microb. Technol. 15:208-214, 1993; Fan and Lee, Biochemical Engineering J. 8:157-164, 2001; Arcuri et al., J. Molecular Catalysis B 21:107-111, 2003; Arcuri et al., Amino Acids 19:477-482, 2000). It has been developed to the stage where commercial processes now operate at scale for the production of specific D-(R)-amino acids (R)-3 using this technology. A key aspect of these processes is the in situ racemisation of the unreacted enantiomer (S)-1 together with carbamoylase catalysed hydrolysis of (R)-2 leading to a dynamic kinetic resolution (DKR) reaction.

Kinetic resolution occurs when an enzyme turns over one enantiomer faster than the other. However, the maximum yield for this type of reaction is only 50%, and the products need to be separated from the starting material. In a dynamic kinetic resolution the enantiomers are racemized, so that (R)- and (S)-enantiomers form a chemical equilibrium and readily interconvert. When the faster reacting enantiomer is converted to the corresponding product, it is replenished due to the racemisation, thereby allowing yields of up to 100%.
In contrast to the enantioselective hydrolysis of racemic 5-substituted hydantoins, the possibility of carrying out enantioselective hydrolysis of 6-substituted dihydrouracils (+/−)-4 (cf. Scheme 2) to their corresponding N-carbamoyl derivatives (R or S)-5, as a route to β-amino acids (R or S)-6, has received very little attention. Syldatk et al., in 1998 (May, O.; Siemann, M.; Pietzsch, M.; Kiess, M.; Mattes, R.; Syldatk, C. J. Biotechnol. 61:1-13, 1998) reported the use of a hydantoinase from Arthrobacter aurescens for the hydrolysis of dihydrouracil ((+/−)-4, wherein R stands for H) and subsequently in 2003 described in a poster that this hydantoinase could be applied to the resolution of 6-phenyldihydrouracil (6-PDHU, (+/−)-4, wherein R stands for phenyl) although poor enantioselectivity and low reaction rates relative to 5-phenylhydantoin ((R)-1 and (S)-1, respectively, wherein R stands for Ph in Scheme 1) were observed.
The Japanese patent application JP06261787 reported enantiomeric excess rates of up to 51% for the hydrolysis of 6-PDHU using Pseudomonas putida IFO 12996; better selectivities (up to 93% of enantiomeric excess) were obtained with substrates containing 6-alkyl substituents. Clearly, there is need for improved methods for the biocatalytic production of β-amino acid precursors or β-amino acids.